Adelaide teen 'off his head' when he died

An Adelaide teenager "off his head on crack" who crashed a stolen car and later died in a drain after being pursued by police had detailed his plans to evade authorities to his sister, a coroner has been told.

Lochlan Brodie Mortimer-Shearer, 18, died in 2016 after he and a friend stole two Nissan Skylines and led police on a chase at speeds of more than 160 km/h.

Coroner David Whittle opened an inquest into his death in the South Australian Coroner's Court on Tuesday.

Court documents say Mr Mortimer-Shearer crashed into a divider on the Port Expressway, drove on the wrong side of the road and eventually collided with the Salisbury Highway Bridge.

The teenager's body was discovered 24 days after the crash by a worker at the Dry Creek salt fields, about 500 metres from the abandoned car.

Submerged in a drain, he was face down in about 15 centimetres of water.

"It is unclear exactly how Mr Mortimer-Shearer got to the point at which he was located," counsel assisting Stephen Plummer said in his opening.

"The logical conclusion is that (he) fled from the stolen Nissan Skyline into the salt fields after the collision."

A statement from Mr Mortimer-Shearer's sister described her brother as "off his head on crack" on the night of the crash.

"He was scaring me ... he was pretty bad and was buzzing," the statement said.

Her brother had previously told her that if he was in a police pursuit, he would crash somewhere he could run from police and step in water so tracker dogs could not pick up his scent.

He had also mentioned that if he was being chased by the police helicopter he would jump into water to cool his body to evade the heat sensors.

The coroner was told Mr Mortimer-Shearer had a bad reaction to amphetamines and fell into the water, before drowning or dying of hypothermia or sudden cardiac arrhythmia.

Mr Whittle was not asked to make any recommendations and will hand down his findings on a date to be set.